So somehow they seem to think that there must be a discontinuity between category 'A' and category 'B' just because they have different names. That this doesn't have to be so is obvious in the case of the visible spectrum but why they have this difficulty to see that this is the same principle for a population changing over time is beyond me.
The problem is, they generally have no direct experience of the natural world. If you watch birds, for example, you're well aware species like the northern junco or eastern meadowlark vary continuously with geographical location across the continent. And if a species can vary with location, why can't it vary with time? But if you've never looked at juncos, or meadowlarks (or many types of warblers, or the hundreds of other species that show clinal variation) you might well think species are somehow well-defined and immutable, rather than just convenient categories.